The goal of this project is to characterize the chronic toxicity and carcinogenicity of multiwalled carbon nanotubes (MWCNT). MWCNT have a wide variety of industrial applications and the use is increasing rapidly. Two-year studies, which include prechronic interim necropsies and immunotoxicity evaluations are in the prestart chemistry/exposure phase. Keywords: toxicity, carcinogenicity, inhalation, multiwalled carbon nanotubes, nanotechnology